Thin film polymer solar cells offer the prospect of providing easy, soln.-based processing and low-temp. manufg. in combination with good efficiencies. Recently, bulk heterojunction solar cells based on blends of regioregular poly(3-hexylthiophene) (P3HT) as p-type material and [6,6]-phenyl-C61-butyric acid Me ester (PCBM) as n-type material have attracted considerable interest. The present study shows that alternating ethynylene and 4,3'-dihexyl-2,2'-bithien-5,5'-diyl along the chain as in poly(ethylene bithienylene) (PEBT) lowers the HOMO energy level by 0.3 eV compared to P3HT. When implemented in bulk heterojunction devices with PCBM, PEBT exhibits the expected increase in open-circuit voltage to values slightly over 1.0 V.